1. Field of the Invention
The disclosure relate to an apparatus for driving a light emitting diode (LED) array, and more particularly, to an apparatus for driving an LED array used as a backlight of a liquid crystal display (LCD) device and an LCD device using the same.
2. Discussion of the Related Art
Flat panel display devices are widely used for various electronic devices such as large-size digital televisions (TVs) requiring a reduction in weight and thickness as well as mobile devices which necessarily need a small size and low power. In the flat panel display devices, LCD devices are applied to all electronic products ranging from small-size devices to large-size devices, and thus are being most widely used.
Since a liquid crystal panel applied to LCD devices cannot self-illuminate, a light source called a backlight is disposed at a rear surface of the liquid crystal panel. Light emitted from the backlight is outputted to the outside through a liquid crystal layer and color filter of the liquid crystal panel, thereby enabling an image to be displayed by the liquid crystal panel. Cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs), or the like are being widely used as the backlight.
However, recently, LEDs which are sufficiently high in luminance and are low in power consumption and manufacturing cost were developed to replace the above-described backlights. That is, since a backlight unit using an LED array does not use a high-cost diffusive film and does not need an inverter unlike fluorescent lamps, the manufacturing cost can be saved, a panel can be easily enlarged in size, a thickness and weight can be reduced, and power consumption and environmental problems can decrease. Accordingly, the backlight unit using the LED array is being widely used.
A backlight unit using LEDs includes an LED array having a plurality of LED strings, each of which is configured with a plurality of LEDs. An amount of light emitted from an LED is proportional to a level of a driving current passing through an active layer of the LED. Driving of the LED denotes supplying and controlling a driving current which flows to the LED.
FIG. 1 is an example diagram illustrating a configuration of a related art apparatus for driving an LED array 60. A related art apparatus 50 for driving an LED array 60, as illustrated in FIG. 1, includes: a control unit 51 that supplies power to an LED array 60 by using an input voltage VIN received from an external source, and controls the amount of current to flow in the LED array 60; a current fixing unit 52 that provides a certain resistance to the control unit 51 in order for a certain amount of current to be supplied to the LED array 60; and a switching unit 53 that controls current flowing in the LED array 60 to vary a luminance of the LED array 60.
The control unit 51 is generally configured as an integrated circuit (IC), and may be mounted on a main board of an LCD device together with a timing controller. The switching unit (current varying unit) 53 may be provided inside or outside the control unit 51. The current fixing unit 52 is disposed outside the control unit 51, and set to have a certain resistance value.
The related art apparatus 50 for driving the LED array controls the luminance of the LED array 60, for controlling the power consumption of the LED array 60. To this end, first, the current fixing unit 52 is set to have an impedance value, which controls the maximum current supplied to the LED array 60 and thus enables the LED array 60 to output a luminance of 100%. Therefore, the control unit 51 continuously supplies a current having the maximum value to the LED array 60 by using the resistance value set by the current fixing unit 52 and the input voltage VIN supplied from an external system.
The control unit 51 includes a power supplier 51a for supplying a voltage and a current to the LED array 60 by using the input voltage VIN and the maximum current. A current supplied to the LED array 60 is fixed to the maximum value by the current fixing unit 52, and the LED array 60 outputs the maximum luminance with the maximum current.
When the LED array 60 outputs light having the maximum luminance, an image displayed by a panel can be preferably sharpened. However, when the LED array 60 outputs light having the maximum luminance, energy consumption increases proportionally. In order to decrease the amount of consumed energy, the control unit 51 generally uses a pulse width modulation (PWM) signal received from the external system. For this purpose, the control unit 51 includes a current amount controller 51b. 
The PWM signal is composed of pulses that have a constant cycle but vary in pulse width, and a turn-on time of a thin film transistor (TFT) configuring the switching unit 53 is controlled by the PWM signal. The time during which the maximum current flows to the LED array 60 is controlled by controlling the turn-on time of the TFT, and thus, a luminance of the LED array 60 may be changed. For example, in dividing one pulse width by a cycle ratio, a high signal occupies 10% and a low signal occupies 90% when a ratio of 10%, and when a ratio of 70%, the high signal occupies 70% and the low signal occupies 30%.
Specifically, when the PWM signal is generated at the ratio of 10%, a high signal having a narrow pulse width is generated, and inputted to the switching unit 53 through the current amount controller 51b. In this case, since the pulse width of the high signal is narrow, a time for which the TFT of the switching unit 53 is turned on is shortened, and thus, a time for which a current flows to the LED array 60 is shortened. Therefore, the luminance of the LED array 60 decreases.
On the other hand, when the PWM signal is generated at the ratio of 70%, a high signal having a broad pulse width is generated, and inputted to the switching unit 53 through the current amount controller 51b. In this case, since the pulse width of the high signal is broad, a time for which the TFT of the switching unit 53 is turned on is extended, and thus, a time for which a current flows to the LED array 60 is extended. Therefore, the luminance of the LED array 60 increases.
Manufacturers of various electronic products such as notebook computers, TVs, tablet personal computers (PCs), etc., by using the above-described LCD device, may adjust the amplitude of the pulse width of the PWM signal sent to the control unit 51 to adjust the amount of power consumed by the LCD device and electronic product.
In order to drive the LCD device and an electronic product with low power, a PWM signal having a narrow pulse width may be received at the current amount control unit 51b from the external system. Conversely, in order to drive the LCD device and an electronic product with high power so as to display a clear image, a PWM signal having a broad pulse width may be received at the current amount control unit 51b from the external system. Accordingly, electronic products vary the pulse width of the PWM signal, and thus drive the LCD devices with desired power.
However, the above-described LCD device of the related art has the following problems. First, the PWM signal must be received from the external system to the control unit 51, for adjusting the luminance of the LED array 60. Therefore, it is difficult for a manufacturer, which is unable to directly develop a PWM signal, to manufacture an electronic product by using the related art LCD device.
Second, the PWM signal for adjusting the luminance of the LED array 60 is required to be synchronized with the frame rate of the LCD device so as to prevent a wavy noise. Therefore, the external system which is driven in connection with the related art LCD device needs to include separate circuits for generating the PWM signal synchronized with the frame rate. For this reason, there is a limitation in developing and manufacturing the external system.